The present invention relates to a flat panel X-ray detector.
In recent years, preparation of data base on the medical data on a patient is being promoted in a medical field in order to perform the medical treatment promptly and appropriately. It should be noted in this connection that the patient utilizes in general a plurality of medical organizations. Therefore, if there is no data prepared in another medical organization, there is possibility that an appropriate medical treatment is not performed.
The data base preparation is also required in respect of the image data of the X-ray photography, and it is hoped from this point that a digital system be developed in respect of the X-ray photograph. It was customary in the past to use a silver halide film in the medical X-ray diagnostic apparatus. For employing a digital system in the medical X-ray diagnostic apparatus, it was necessary to develop the photograph film image and scan again the developed film image with a scanner, which was laborious and time-consuming.
In recent years, a system of directly converting the image into digital data has been realized by using the CCD camera sized about one inch. However, in photographing, for example, a lung, a region of about 40 cmxc3x9740 cm is photographed, making it necessary to use an optical apparatus for collecting light, leading to the problem that the apparatus is rendered bulky.
As a system for overcoming the above-noted problems inherent in the two systems described above, proposed is a flat panel X-ray detector of an indirect conversion system using an amorphous silicon thin film transistor (a-Si TFT). FIG. 1 shows the circuit construction of the flat panel X-ray detector. The operation of the flat panel X-ray detector will now be described with reference to FIG. 1.
The flat panel X-ray detector shown in FIG. 1 is a flat panel X-ray detector of an indirect conversion type, in which an incident X-ray is converted into a visible light by, for example, a phosphor, and the converted visible light is further converted into an electric charge for each pixel by a photo-conduction film.
As shown in FIG. 1, the flat panel X-ray detector comprises pixels e (i, j) (i=1 to 2000, j=1 to 2000). Each pixel e comprises a switching TFT 401 formed of a-Si, a photo-conduction film 402 and a Cst 403. These pixels e are arranged to form an array, the row of the array consisting of hundreds of to thousands of pixels e and the column of the array also consisting of hundreds of to thousands of pixels. A negative bias voltage is applied from a power source 404 to the photo-conduction film 402. The switching TFT 401 is connected to a signal line 405 and to a scanning line 406 and is subjected to an on-off control by a scanning line driving circuit 407. The terminal of the signal line 405 is connected to an amplifier 410 for the signal detection via a change-over switch 409 that is controlled by a signal line control circuit 408.
If an X-ray is incident, the phosphor (not shown) irradiated with the X-ray emits a fluorescent light. The fluorescent light is then converted into an electric charge by the photo-conduction film 402, and the electric charge is accumulated in the Cst 403. If a scanning line 406 is driven by the scanning line driving circuit 407 so as to turn on a column of switching TFTs 401 connected to one of the scanning lines 406, the accumulated charge is transferred through the signal line 405 toward the amplifier 410. By the change-over switch 409, the charge is supplied to the amplifier 410 for each pixel so as to be converted into a dot sequential signal.
The amount of the electric charge differs depending on the amount of light incident on the pixels (i, j) so as to change the output amplitude of the amplifier 410. By subjecting the output signal of the amplifier 410 to an A/D conversion, the electric charge can be converted directly into a digital image. Further, the pixel region can be made thin and large by utilizing the array of the switching TFTs 401.
FIG. 2 is a plan view showing the construction of the pixel 501 included in the flat panel X-ray detector. As shown in the drawing, the pixel 501 comprises a switching TFT 401 for the reading, a Cst 403, a Cst line 502 connected to the Cst 403, an auxiliary electrode 503 facing the Cst 403, a pixel electrode 504, a signal line 405, and a scanning line 406. A contact portion 505 is formed in each of the switching TFT 401 and the auxiliary electrode 503.
It should be noted that the layers above the pixel electrode 504 and the region outside the pixel 501 are omitted from the drawing of FIG. 2. Incidentally, it is possible to utilize the floating capacitance of the other elements and the wiring in place of arranging the Cst 403.
FIG. 3 is a cross sectional view along the line IIxe2x80x94II shown in FIG. 2, which shows the constructions of the layers formed above the pixel electrode 504.
As shown in FIG. 3, a pixel electrode 504, a p-type contact film 601, a photo-conduction film 402, an n-type contact film 602, a common electrode 603, a phosphor layer 604 and a reflective layer 605 are laminated in the order mentioned on the structure including the switching TFT 401, the Cst 403, the auxiliary electrode 503, the signal line 405 and the scanning line (not shown).
If an X-ray is incident on the phosphor layer 604 through the reflective layer 605, a fluorescent light is emitted from the phosphor layer 604 irradiated with the X-ray, and the fluorescent light thus emitted is scattered. The fluorescent light then enters the photo-conduction film 402 directly or is reflected from the reflective layer 605 and, then, the reflected fluorescent light enters the photo-conduction film 402. In the photo-conduction film 402, the fluorescent light is converted into an electric charge. It should be noted that, since voltage is applied across the photo-conduction film 402, the generated electric charge is attracted by the pixel electrode 504 for each pixel 501 so as to be accumulated in the Cst 403 through the pixel electrode 504.
In the flat panel X-ray detector of the construction described above, a fluorescent light is emitted in every direction from the phosphor layer 604 upon irradiation with the X-ray. The fluorescent light thus emitted is scattered and reflected from the reflective layer 605. It follows that it is highly possible for the fluorescent light emitted from the phosphor layer 604 in a certain pixel to arrive at the photo-conduction film 402 of the adjacent pixel. It should be noted that voltage is applied to the photo-conduction film 402 and, thus, the electric charge converted from the fluorescent light is scarcely scattered so as to arrive at the pixel electrode 504 corresponding to the particular region. Also, there is a problem that the light emitted from the phosphor is attenuated by the absorption within the phosphor film and by the reflection from the upper surface and the bottom surface of the film so as to lower the efficiency.
As a result, the fluorescent light emitted from the phosphor layer 604 is scattered so as to arrive at the adjacent pixel. The fluorescent light is converted into an electric charge in the photo-conduction film 402 of the adjacent pixel, and the electric charge thus generated is accumulated in the pixel electrode 504 of the adjacent pixel. It follows that a problem is generated that the resolution is deteriorated.
An object of the present invention is to provide a flat panel X-ray detector, which has a high resolution and permits manufacturing a large apparatus with a low cost.
According to the present invention, there is provided a flat panel X-ray detector, comprising an X-ray-electric charge conversion film converting an incident X-ray into an electric charge, a pixel electrode contiguous to the X-ray-electric charge conversion film and arranged for every pixel, a switching element connected to the pixel electrode, a signal line connected to the switching element, and a scanning line supplying a driving signal to the switching element, wherein the X-ray-electric charge conversion film contains phosphor particles, a photosensitive material, and a carrier transfer material.
According to the present invention, there is provided an X-ray-electric charge conversion film, containing a photosensitive material, phosphor particles covered with the photosensitive material, and a carrier transfer material.